10.13 GNUpatch and Traditional patch

The current version of GNUpatch normally follows the
POSIX standard. See patch and POSIX, for the few exceptions
to this general rule.

Unfortunately, POSIX redefined the behavior of patch in
several important ways. You should be aware of the following
differences if you must interoperate with traditional patch,
or with GNUpatch version 2.1 and earlier.

In traditional patch, the -p option's operand was
optional, and a bare -p was equivalent to -p0. The
-p option now requires an operand, and -p 0 is now
equivalent to -p0. For maximum compatibility, use options
like -p0 and -p1.

Also, traditional patch simply counted slashes when
stripping path prefixes; patch now counts pathname
components. That is, a sequence of one or more adjacent slashes now
counts as a single slash. For maximum portability, avoid sending
patches containing // in file names.

In traditional patch, backups were enabled by default. This
behavior is now enabled with the --backup (-b)
option.

Conversely, in POSIXpatch, backups are never made,
even when there is a mismatch. In GNUpatch, this
behavior is enabled with the --no-backup-if-mismatch option,
or by conforming to POSIX.

The -b suffix option of traditional patch is
equivalent to the ‘-b -z suffix’ options of GNUpatch.

Traditional patch used a complicated (and incompletely
documented) method to intuit the name of the file to be patched from
the patch header. This method did not conform to POSIX, and had
a few gotchas. Now patch uses a different, equally
complicated (but better documented) method that is optionally
POSIX-conforming; we hope it has fewer gotchas. The two methods
are compatible if the file names in the context diff header and the
‘Index:’ line are all identical after prefix-stripping. Your
patch is normally compatible if each header's file names all contain
the same number of slashes.

When traditional patch asked the user a question, it sent
the question to standard error and looked for an answer from the first
file in the following list that was a terminal: standard error,
standard output, /dev/tty, and standard input. Now
patch sends questions to standard output and gets answers
from /dev/tty. Defaults for some answers have been changed so
that patch never goes into an infinite loop when using
default answers.

Traditional patch exited with a status value that counted
the number of bad hunks, or with status 1 if there was real trouble.
Now patch exits with status 1 if some hunks failed, or with
2 if there was real trouble.

Limit yourself to the following options when sending instructions
meant to be executed by anyone running GNUpatch,
traditional patch, or a patch that conforms to
POSIX. Spaces are significant in the following list, and
operands are required.